Photothermographic (i.e., heat developable photographic) imaging materials that are classified as "dry silver" compositions or emulsions comprise a light insensitive, reducible silver source, a light sensitive material which generates silver (i.e., a Ag.degree. latent image) when irradiated, and a reducing agent for the reducible silver source. The light sensitive material is generally photographic silver halide, which must be in catalytic proximity to the light insensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source by the reducing agent. It has been long understood that silver (Ag.degree.) is a catalyst for the reduction of silver ions and the silver-generating light sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the reducible silver source with a halogen-containing source (e.g., U.S. Pat. No. 3,457,075), coprecipitation of silver halide and reducible silver source material (e.g., U.S. Pat. No. 3,839,049), and other methods that intimately associate the silver halide and the silver source.
The reducible silver source is a material that contains silver ions. The preferred reducible silver source comprises silver salts of long chain aliphatic carboxylic acids, usually having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight have been primarily used. Salts of other organic acids or other organic materials, such as silver imidazolates have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as image source materials.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the light sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver ions, which are in catalytic proximity to the silver halide grains bearing the latent image.
There are several imaging systems in which the thermal reduction of silver ion to silver metal by means of an oxidisable developer generates a colour image.
One conventional way of obtaining colour images from photothermographic emulsions is by the inclusion of dye forming materials into the emulsion. Upon imaging and thermal development, a compound is oxidised to form a dye and a reduced silver image is simultaneously formed in the exposed region. In this way a dye enhanced silver image can be produced, as shown for example in U.S. Pat. Nos. 3,531,286, 4,187,108, 4,426,441, and 4,460,681.
Dye-containing photothermographic systems in which the reactants and products remain in contact after imaging can result in several problems. For example, turbid and hazy colour images are often formed due to dye contamination with the reduced metallic silver image in the exposed area of the material after thermal development. Additionally, the resulting prints tend to develop colour in the non-imaged background. This "background stain" is caused by slow oxidation of dye precursors during storage.
In the case of purely thermographic media, image-wise application of heat generates the image and light-sensitive silver halide is not generally present. Those systems also suffer from the problem that the presence of a black silver image in addition to the desired colour dye image may lead to turbid or `muddy` images.
One method of overcoming this problem is to transfer the dye image to a receptor or a receiving layer remote from the layer(s) containing silver. However, to provide good colour separation and background stability it is important to be able to transfer the dye image without transfer of other components, e.g., leuco dyes which may air-oxidise in the receptor and cause stain.
An attractive solution is to design the development chemistry such that the coloured species constituting the desired image is cleaved from a developer molecule in the redox development process in a form that is susceptible to thermal diffusion to a receiving layer remote from the layer(s) containing the silver.
The redox cleavage may be mediated directly by interaction with the silver salt or indirectly, by means of a "cross-oxidising agent". The latter is a compound capable of being oxidised by a silver salt and whose oxidised form participates in the redox reaction which results in the release of the dye.
British Patent No. 2100458 discloses the use of sulphonamidophenol and sulphonamidonaphthol dye-releasing redox compounds which release a diffusible dye on heat development. British Patent No. 2100016 discloses the use of dye-releasing couplers which, in combination with a reducing agent, release a diffusible dye on heat development. Various other dye-releasing systems have been disclosed, e.g., U.S. Pat. Nos. 4,060,420, 4,088,469, 4,499,180, 4,511,650, and 4,731,321, often involving thermal generation of a basic substance.
An example of suitable redox cleavage chemistry is the so-called "preformed dye release" disclosed in U.S. Pat. No. 4,981,775, in which a dye chromophore is linked to a developer moiety by a carbonyl group in a manner such that the dye is released upon oxidation of the developer and may be sublimed to a separate receptor sheet. In one example, the developer moiety is equipped with long alkyl chain substituents for ballasting purposes in order to promote selective sublimation of the released dye.
Another example of preformed dye release is the chemistry disclosed in U.S. Pat. No. 4,463,079, which discloses the use in non-aqueous photothermographic media compounds of formula: EQU R--SO.sub.2 --D
in which;
R represents a reducing group capable of being oxidised by an organic silver salt and D represents an image forming dye part. In all the examples given, the bond between R and SO.sub.2 forms a sulphonamide group.
Similar compounds are disclosed in U.S. Pat. No. 4,740,455 and references cited therein and European Pat. No. 385496 in the context of aqueous-coated photothermographic media. Base is normally required to effect the dye release.
Japanese Patent Application No. 62-065037 discloses heat-developing photosensitive media comprising silver halide, binder, reducing agent and a dye donative compound which is a polymer derived from a monomer of formula: EQU Q--X--SO.sub.2 -Dye
in which;
Q is an ethylenically unsaturated group,
X is an oxidisable group and,
Dye is the residue of an image forming dye formed by the extraction of a hydrogen atom.
Following exposure, thermal development causes release of a diffusible dye. The media disclosed are aqueous based, and do not contain silver salts other than photosensitive silver halide.
U.S. Pat. No. 4,713,316 discloses the use in photothermographic media of dye providing substances of formula: EQU (Dye--X.paren close-st.Y.sub.q
in which;
Dye represents a dye which becomes mobile when released from the molecule of the above formula,
X represents a bond or linking group,
Y represents a group which releases Dye in correspondence or counter correspondence to light sensitive silver salts having a latent image distributed imagewise, and
q is 1 or 2.
The media preferably comprise an organic silver salt oxidising agent in addition to light sensitive silver halide, but only aqueous based systems are disclosed, and no advantage is taught for compounds in which q is 2.
U.S. Pat. No. 4,021,240 discloses a different colour-forming chemistry in which a p-sulphonamidophenol developer is oxidised by silver ion and then couples with a "four-equivalent" colour coupler to form a precursor which releases a dye by elimination of sulphinic acid.
A further colour-forming chemistry is disclosed in European Patent No. 177328, U.S. Pat. No. 4,622,395, and U.S. Pat. No. 4,594,307 which describe acylated phenazine, phenoxazine and phenothiazine leuco dyes. Oxidation of these leuco dyes by silver ion causes release of the corresponding azine dyes via cleavage of ##STR1## bond.
A problem shared by all of these prior art approaches is that of unwanted transfer of coloured or colour-generation materials in the non-image areas, causing unacceptably high Dmin's either immediately or on storage. This is a result of insufficient mobility differential between the released dye and its precursors. A related problem arises when the media comprises two or more colour-forming layers, as in a full-colour imaging element. Unwanted migration of the colour forming compounds among the various layers can occur, particularly at the coating stage, leading to crosstalk and inaccurate colour rendition.
Various attempts have been made to reduce the mobility of the colour releasing compounds including the use of bulky alkyl ballasting groups, as disclosed in U.S. Pat. No. 4,981,775, and the use of hydrophillic groups is disclosed in U.S. Pat. No. 4,463,079. However, none of the known techniques has proved to be fully effective, especially in the context of hydrophobic media, coated from non-aqueous solvents. Controlling the diffusion of colour forming or releasing compounds is relatively easy in aqueous based systems (both wet and dry developed) owing to the special properties of the binder involved, normally gelatin or similar hydrophillic colloids. These binders form stable dispersions of the essentially hydrophobic colour forming chemistry, and because of the incompatibility of the two phases, diffusion of the colour forming chemistry within the hydrophillic binder is limited. It is common practice to heighten this incompatibility by incorporating bulky hydrophobic groups in the colour forming compounds, or by attaching the colour forming compounds to hydrophobic polymer backbones, as is frequently done with colour couplers in conventional colour negative film. However, this biphasic approach is very difficult in the case of non-aqueous based media, where the binders used are essentially hydrophobic and cannot easily be made to be incompatible with the colour generating chemistry.
U.K. Patent Application No. 2198139 discloses polymeric leuco dyes for use in heat sensitive or light and pressure sensitive recording materials. The polymeric leucos become coloured on contact with electron accepting (acidic) compounds, but do not release a diffusible dye.
Likewise, Japanese Patent Application No. 50-020809 discloses compounds formed by the reaction between bis (acid chlorides) and reduced diazine, oxazine and thiazine dyes for use in heat or pressure sensitive media. There is no disclosure of diffusible dye release or imaging with silver salts.
It has now been found that substantial improvements in non-aqueous thermographic and photothermographic media are possible when two or more dye releasing moieties are linked together in a single molecule.